Reference is made to U.S. Pat. No. 5,418,431, assigned to the assignee of the present invention, illustrating a radio frequency (RF) plasma source. The previous design is for a lightweight, low power plasma source that produces a chemically reactive plasma that is capable of cleaning contamination from thermal radiators of spacecraft. The device produces ions having energy less than 20 eV. At this energy level, there is little risk of having the ions damage delicate substrate during a cleaning operation. The cleaning rate is such that operating the source a few times per month effectively prevents radiators from degrading significantly. In the previous system, one cleaner is mounted on each solar wing of a spacecraft; and the system weighs approximately 14 pounds.
The earlier design does not include a diffuser at the gas inlet. Without such an arrangement, the density in the plasma generation tube peaks along a longitudinal central axis of the tube. This is a disadvantage because RF waves from an associated antenna are able to dissociate only a small fraction of inlet gas into reactive species. The physics of the dissociation process is not fully understood, but recent research indicates that most high energy electrons in a helicon-type device such as the compact plasma cleaner are formed near the antenna around the plasma generation tube. High energy electrons sustain the plasma and are responsible for the dissociation reactions.
Without a diffuser, the gas density is lowest along the walls of the plasma generation tube where the high energy electrons are generated. By adding a diffuser the gas density is more uniform across the plasma generation tube. Therefore, more molecules are available near the plasma generation tube walls; and more high energy electrons (and therefore plasma) are produced.
In the prior design the plasma generation tube is formed of fused silica glass, and a semirigid coaxial cable inserted through a source housing supports the helical antenna around the tube. Because the plasma generation tube is silica, the antenna cannot be brazed to it. An antenna feed itself supplies the necessary external support for the antenna. Because of the external support, the antenna is larger than electrically necessary. The extra supporting material of the antenna and feed adds mass to the system. Securing a thinner antenna directly to the plasma generation tube and allowing the tube to support the antenna is a weight-saving solution.
The previously used feed includes a coaxial cable employing a teflon dielectric separating two coaxial conductors, and it presents an unbalanced feed to the balanced antenna design with no provision for cancelling out-of-balance ground currents. The teflon dielectric, while low-loss, represents a potential problem in that it may deteriorate due to proton bombardment while in geosynchronous orbit, altering the impedance match over time. The new strip transmission line design eliminates the need for a supporting dielectric, avoiding these potential problems.
In the prior design the structure of the plasma source relied on brittle magnets, held in compression, for support. Furthermore, the design does not consider thermal expansion and shock effects. External structure assists the plasma source in surviving loads due to launch, solar panel deployment and pyroshock.
Ground-based reactive-plasma sources used for microelectronics processing are known in the literature. They differ from the present invention in two key respects, however. First, their size, mass, and power and gas consumption disqualify them from practical use in space applications. Second, most produce ions having sufficient energies to sputter many materials. This represents an important prohibition, especially for optical surface cleaning applications.
While the prior techniques function with a certain degree of efficiency, none disclose the advantages of the improved plasma source of the present invention as is hereinafter more fully described.